US 20060198342 A1 Abstract e
^{j(2nn/N) }calculating section 101 generates a bth chip C(a,b) of an ath spreading code based on C(a,b)=e^{j(2nn/N) }where e is a base of natural logarithm and N is a length of the spreading code (i.e. spreading code length). It is assumed in the above equation that n=a×b, a=0˜N−1, and b=0˜N−1. It is thereby possible to generate orthogonal spreading codes with arbitrary lengths. Claims(11) 1. A method of generating spreading codes generating a bth chip C(a,b) of an ath spreading code by a following equation, assuming that e is a base of natural logarithm and that N is a length of the spreading code: C(a,b)=e ^{j(2nn/N) } (1) where n=a×b, a=0˜N−1, and b=0˜N−1.
2. The method of generating spreading codes according to 3. A CDMA transmission apparatus comprising:
a spreading code generator that generates a bth chip C(a,b) of an ath spreading code by a following equation, assuming that e is a base of natural logarithm and that N is a length of the spreading code: C(a,b)=e ^{j(2nn/N) } (1) where n=a×b, a=0˜N−1, and b=0˜N−1; and a spreader that spreads a transmission signal using the spreading code generated in the spreading code generator. 4. The CDMA transmission apparatus according to 5. The CDMA transmission apparatus according to 6. The CDMA transmission apparatus according to 7. The CDMA transmission apparatus according to 8. The CDMA transmission apparatus according to 9. A CDMA reception apparatus comprising: a spreading code generator that generates a bth chip C(a,b) of an ath spreading code by a following equation, assuming that e is a base of natural logarithm and that N is a length of the spreading code: C*(a,b)=e ^{−j(2nn/N) } (3) where n=a×b, a=0˜N−1, and b=0˜N−1; and
a despreader that despreads a received signal using the spreading code generated in the spreading code generator.
10. The CDMA reception apparatus according to 11. The CDMA reception apparatus according to Description The present invention relates to a method of generating spreading codes, CDMA transmission apparatus and CDMA reception apparatus, and more particularly, to a method of generating orthogonal spreading codes, and a CDMA transmission apparatus and CDMA reception apparatus using the orthogonal spreading codes. Conventionally, in CDMA communication systems, the transmission side spreads a transmission symbol with a spreading code, and the reception side obtains a reception symbol by despreading with the same spreading code. At this point, to prevent interference of signals among channels, it is general to use spreading codes orthogonal to one another among the channels. A method of generating the orthogonal codes is disclosed in JP H12-115130, for example. Whether or not the orthogonality is maintained among spreading codes largely affects the communication quality in CDMA communications. Therefore, when the synchronization between spreading codes and similarity of channels is ensured, orthogonal codes are generally used. Actually, all the spreading codes are made synchronized completely on downlink, and even when the multipath exists, it is ensured that all the spreading codes are transmitted on the same channel. Devices are sometimes made for the ensuring on uplink. Therefore, in CDMA communications, how many more orthogonal codes can be generated efficiently affects the system communication capacity (the number of channels) greatly. Symbol copy section As a result, multiplying section The I component and Q component thus spread in spreading section CDMA reception apparatus Spreading codes generated in spreading code generating section Inter-symbol adding section Demapping section However, since a sequence of “1, −1” (i.e. either numeric value 1 or −1) is used for spreading codes in conventional CDMA communications, it is inevitable that the length of the code becomes powers of two to generate orthogonal codes having high use efficiency. This imposes a significant restriction on determining parameters of the system such as the length of a frame and a basic clock. For example, when the basic clock is made coincident with that in another system, the design becomes very difficult. Actually, in W-CDMA specified in 3GPP (3rd Generation Partnership Project), the restriction limits the chip rate to 3.84 Mcps, and similarly, in cdma 2000 specified in 3GPP2, the chip rate is 1.228 Mcps. It is an object of the present invention to provide a method of generating spreading codes, and CDMA transmission and reception apparatuses enabling generation of spreading codes with arbitrary lengths. This object is achieved by generating a bth chip C(a,b) of an ath spreading code by the following equation where e is a base of natural logarithm and N is a length of a spreading code (i.e. spreading code length):
It is thereby possible to generate spreading codes with orthogonality and arbitrary spreading code lengths N (N is any natural numbers). Embodiments of the present invention will specifically be described below with reference to accompanying drawings. In subsequent step ST Next, spreading code generating apparatus When the spreading code with the code length N is generated on the code number a, a positive result is obtained in step ST The aforementioned processing is performed until the code number a is N−1 and a positive result is obtained in step ST Thus, according to spreading code generating apparatus In CDMA transmission apparatus Symbol copy section Spreading code generating section Actually, the spreading code e Thus, spreading section Spreading codes generated in spreading code generating section As a result, a spreading code e Multiplying section Thus, despreading section The reason will be described below why orthogonality is ensured among codes, and symbols spread at the transmission side can be divided with excellence at the reception side, by using spreading codes e First, it is assumed that an ith symbol is S(a,i) that is spread with a spreading code e A received signal R (a, i, b) of an ith symbol that is spread with the spreading code C(a, b)=e The following equation expresses a despreading result Q(a, i) obtained by despreading the received signal R(a, i, b) with the spreading code C*(a, b)=e As a result, as can be seen from Eq. (6), it is possible to extract a signal spread with the ath spreading code with accuracy at the reception side by the spreading code that is a complex conjugate of the ath spreading code. In contrast thereto, the signal spread with the ath spreading code cannot be extracted by despreading using a spreading code (for example, a+1th spreading code) that is different from the ath spreading code. This will be described below. The case of N=3 is taken as an example. In this case, spreading code generating section Spreading code 0 (a=0): [1, 1, 1] Spreading code 1 (a=1): [1, e Spreading code 2 (a=2): [1, e As an example, considering the case where a signal spread with spreading code 1 is despread with a complex conjugate of spreading code 2, a despreading result Q(2, i) is as expressed in the following equation:
As can be seen from Eq. (7), when the despreading processing is performed using a spreading code different from that when spreading, the despreading result is 0. In other words, it is understandable that spreading codes generated in spreading code generating section Referring to As shown in In contrast thereto, as shown in Thus, according to this Embodiment, since spreading codes are generated as expressed in Eq. (1), it is possible to generate spreading codes with arbitrary lengths orthogonal to one another. It is thereby possible to make it easy to design a CDMA system, for example. In addition, the method of generating spreading codes of the invention is similar to the case of generating each orthogonal signal in OFDM. Subcarriers orthogonal to one another are generated in OFDM. The spreading codes generated in the invention correspond to 0 Hz to (N−1) Hz in OFDM. However, the actual use and effects are greatly different from those in OFDM, and it is possible to obtain processing that cannot be implemented in OFDM and specific effects that cannot be obtained in OFDM, by using as spreading codes. The differences from OFDM will be described below. When N subcarriers are generated in OFDM, the same signal sequence as C(a,b) shown in Eq. (1) is used as a time waveform. However, the time waveform itself transmitted to air is generated by such a method in OFDM. In other words, a signal generated in OFDM is a continuous time waveform, and when the continuity is interrupted, the transmission spectrum is significantly affected, and spreading of the spectrum out of the band become extremely large. Therefore, in OFDM, only extremely minute processing such as filtering that limits the entire band can be performed on the time waveform in a range of FFT. Extensive processing causes significant deterioration in performance and in transmission spectrum. In contrast thereto, it is fundamental in the invention using spreading codes generated in Eq. (1) in CDMA, and the invention enables chips subsequent to spreading to be treated independently. In other words, it is possible to provide chips with any processing (process). For example, chip may be provided with some processing as performed in general CDMA such as band limitations for chip with a root Nyquist filter, unequal chip intervals, insertion of an chip of another signal (such as a pilot signal and control signal) in between chips, change in the chip order and scrambling. In particular, by using a method of changing the order of chips as described later in Embodiment 2 or a method of performing scrambling as described later in Embodiment 3, a new effect is produced that makes it possible to use OFDM used only in an isolated cell in cellular with interference with other cells. In other words, in OFDM, since Eq. (1) is used for generating orthogonal subcarriers, a temporally-continuous time waveform is generated and processing on the waveform is limited to severely restricted processing. In contrast thereto, in the invention, since spreading codes are generated using Eq. (1), various processing can be performed on spread chips. Further, in performing various processing, the code length is not limited to power-of-two, and it is thereby possible to diversify a configuration of an apparatus to perform each processing. As can be seen from the foregoing, the invention features using codes of waveforms similar in OFDM, but is distinguished in subject of use, and results in splendid differences in the effect. In addition, signals cannot be divided for each path in OFDM, but in CDMA where band limitations are performed for chips using a root Nyquist filter and the like, since it is possible to separate paths as in general CDMA, the diversity effect can be obtained by RAKE reception. A feature of this Embodiment is changing the order of chips in spreading codes generated based on Eq. (1). In order to change the order of chips, chip interleaving can be performed after generating spreading codes with a code length N based on Eq. (1), or a spreading code with the order of chips changed can first be generated by performing calculation as shown in the following equation:
By this means, the transmission side is capable of performing the spreading processing on a transmission signal while changing the order of chips of spreading codes with an arbitrarily code length N orthogonal to one another, and the reception side is capable of extracting a desired signal with accuracy from a code division multiplexed signal. Thus, in this Embodiment, in addition to the configuration of Embodiment 1, chips of a spreading code are changed as appropriate, and for example, following effects can be obtained. An effect of averaging the correlation among codes: When a spreading-factor is large, there is a possibility that a channel is changed between the first chip and the last chip, and in this case, the orthogonality among particular codes largely deteriorates and increases the correlation among the codes. Even in such a case, by changing the order of chips, it is possible to average the correlation so that almost the same correlation is obtained among all the spreading codes not depending on the spreading codes. It is thus possible to improve the effect of error correcting code, and make it easy to cancel interference by correlation (despreading). An effect of reducing interference with other cells: By making the order of chips different from those in other cells (assigning different order variation patterns for each cell), it is possible to average the correlation with the other cells, and obtain almost the same effect as in scrambling. An effect of randomizing the influence of multipath: Interference occurs among chips when multipath exists, but randomizing the order of chips randomizes occurrences of interference, and as a result, it is possible to improve the effect of error correcting code and facilitate cancellation of interference by correlation (despreading). In the aforementioned configuration, CDMA transmission apparatus First, it is possible to reduce the correlation among chips. In general CDMA, a phase between chips is 0, n or ±π/4, and a phase difference between chips varies with the spreading code, and the effect of a delay signal differs to some extent for chips and is averaged. In contrast thereto, when spreading codes of the invention are used and rearrangement of chips is assumed not to be performed, since the phase differences between chips are equal in any spreading codes, the effect of a delay signal causes the correlation between chips. Taking this respect into account, it is desirable to randomize phase differences among spreading codes by performing the scrambling processing as in this Embodiment. It is thereby possible to enhance the error correcting capability at the reception side. Further, when the scrambling processing is performed in addition to using spreading codes of the invention as in this Embodiment, it is possible to use codes other than [1, −1] as scrambling codes. In other words, when scrambling codes are generated, it is possible to generate scrambling codes with arbitrary lengths and periods and the order thereof changed by the same method as the method of generating spreading codes as described above. It is thereby possible to set the length of a scrambling code with ease and generate the code readily. Thus, according to this Embodiment, the scrambling processing is performed as well as using spreading codes described in Embodiments 1 and 2, whereby in addition to the advantageous effects of Embodiments 1 and 2, it is possible to further improve error rate characteristics at the reception side, and to increase the number of usable scrambling codes. In addition, while this Embodiment describes the case of multiplying a spread signal by a scrambling code, but the invention is not limited to such a case, and the same advantageous effects as in the aforementioned Embodiment can be obtained in the cases of multiplying a transmission signal prior to spreading or a spreading code itself by a scrambling code. A feature of this Embodiment is generating spreading codes of the invention using a hierarchical code tree. It is thereby possible to generate more spreading codes while maintaining the orthogonality. The code tree has been used conventionally as a method of generating spreading codes with different spreading-factors orthogonal to one another, and for example, OVSF (Orthogonal Variable Spreading-Factor) codes are one example of spreading codes generated using the code tree. In the example of The OVSF codes conventionally used in CDMA are a specific case among the aforementioned codes, and the spreading-factor is always increased by two times. However, the spreading codes of the invention can generate orthogonal codes other than powers of two and therefore, as shown in In addition, by using the code tree, it is possible to generate codes all orthogonal to one another even in codes with different spreading codes (for example, in the example of An advantage will be described below when spreading codes of the invention are generated using the code tree. The effect of generating the spreading codes of the invention using the code tree is the same as in the OVSH codes in such a respect that codes with different spreading-factors can be made orthogonal and multiplexed. However, when the spreading codes of the invention are generated using the code tree, it is possible to generate spreading codes orthogonal to one another in spreading-factors very close to the basic-factor such as 18-factor spreading and 15-factor spreading, if only the basic spreading-factor (3 in the example of For example, with respect to the quality of communications, in the case where 16-factor spreading provides slightly less quality in 16 QAM (Quadrature Amplitude Modulation), while providing excessively adequate quality in QPSK (Quadrature Phase Shift Keying) the use has conventionally been limited to 16 QAM in 32-factor spreading or QPSK in 16-factor spreading. Such use is insufficient in effective use of resources. In contrast thereto, the spreading codes of the invention generated using the code tree enable use of 16QAM in 18-factor spreading (if still insufficient, in 21-factor spreading), while inversely enabling use of QPSK in 15-factor spreading (if allowed more, in 12-factor spreading or 9-factor spreading). Thus, according to this Embodiment, the spreading codes of the invention are generated using a hierarchical code tree, whereby in addition to the advantageous effects of Embodiment 1, it is possible to generate spreading codes with various spreading-factors while maintaining the orthogonality and enabling effective use of resources. This Embodiment proposes performing collectively generation of spreading codes of the invention and spreading processing at a transmission side using an inverse Fourier transformer (IDFT), and further proposes performing collectively generation of spreading codes of the invention and despreading processing at a reception side using a Fourier transformer (DFT). It is thereby possible to reduce amounts of calculation. Three-point IDFT Three-point DFT When generating spreading codes of the invention and performing the spreading or despreading processing using the spreading codes simply, a calculation amount of the order of a square of the number (N) of codes is required, but using IDFT or DFT as in this Embodiment requires a calculation amount of only of the order of log(N). Thus, according to this Embodiment, the spreading processing using the spreading codes of the invention with inverse Fourier transformer In addition, as described in Embodiment 2, when the order of chips is changed at the transmission side, the reception side restores the order of chips and then performs DFT processing (or, input lines to DFT Further, as described in Embodiment 3, when the scrambling processing is performed at the transmission side, descrambling processing and then DFT processing are performed at the reception side. Furthermore, this Embodiment describes the configuration of the case where transmission data # In this Embodiment, as a method of reducing an amount of calculation when transmission and reception is performed using spreading codes with different spreading-factors as in Embodiment 4, proposed are a CDMA transmission apparatus where an inverse discrete Fourier transformer is cascaded as appropriate and a CDMA reception apparatus where a discrete Fourier transformer is cascaded as appropriate. In other words, in using spreading codes with different spreading-factors, IDFT (transmission side) or DFT (reception side) is used hierarchically. Further, in CDMA transmission apparatus Three-point IDFT Thus, by cascading a plurality of IDFTs Five-point DFT Thus, by cascading a plurality of DFTs In addition, CDMA transmission apparatus Further, CDMA reception apparatus Thus, according to this Embodiment, in using spreading codes with different spreading-factors as explained in Embodiment 4, hierarchical spreading processing is performed by cascading inverse discrete Fourier transformers The present invention is not limited to the aforementioned Embodiments, and is capable of being carried into practice with various modifications thereof. In an aspect of a method of generating spreading codes of the invention, a bth chip C(a,b) of an ath spreading code is generated by a following equation when it is assumed that e is a base of natural logarithm and N is a length of the spreading code (i.e. spreading code length):
According to this method, it is possible to generate spreading codes with orthogonality and an arbitrary spreading code length N (N is any natural numbers). In another aspect of the method of generating spreading codes, spreading codes with different spreading code length are generated by successively multiplying N in Eq. (1) by k (k is positive integers). According to this method, it is possible to generate more spreading codes while maintaining the orthogonality. Actually, spreading codes with different spreading code length are generated hierarchically using a code tree, and in the invention, since it is possible to generate orthogonal codes while being not limited to codes of powers of two unlike the conventional case, it is possible to generate spreading codes with arbitrary spreading-factors. As a result, fine adjustments can be made to resources, in addition to further increases in the number of spreading codes. An aspect of a CDMA transmission apparatus of the invention adopts a configuration provided with a spreading code generator that generates a bth chip C(a,b) of an ath spreading code by a following equation when it is assumed that e is a base of natural logarithm and N is a length of the spreading code (i.e. spreading code length):
According to this configuration, it is possible to generate spreading codes with orthogonality and an arbitrary spreading code length N (N is any natural numbers). As a result, by using the CDMA transmission apparatus of the invention, versatility is increased in frame length, basic clock and the like in a system, and it is thereby possible to facilitate the design of the CDMA system. Another aspect of the CDMA transmission apparatus of the invention adopts a configuration where the spreading code generator rearranges the order of chips of the spreading code generated in Eq. (1) for spreading codes. According to this configuration, for example, even when the channel variation occurs at high speed, it is possible to average the correlation among spreading codes, and it is thus possible to enhance the effect of error correcting code and to facilitate cancellation of interference by the correlation (despreading). Another aspect of the CDMA transmission apparatus of the invention adopts a configuration further provided with a scrambler that multiplies the transmission signal, the spreading code or a spread signal by a scrambling code. According to this configuration, in addition to the cell distinguishing effect, the correlation among chips can be reduced, and it is thus possible to further improve error rate characteristics on the reception side. A combination with spreading codes of the invention increases the number of usable scrambling codes. Another aspect of the CDMA transmission apparatus of the invention adopts a configuration where the spreading code generator generates spreading codes with different spreading code length by successively multiplying N in Eq. (1) by k (k is positive integers). According to this configuration, it is possible to generate more spreading codes while maintaining the orthogonality, and to further increase an amount of data concurrently transmissible. Moreover, since orthogonal codes can be generated being not limited to codes of powers of two, it is possible to generate spreading codes with arbitrary spreading-factors. As a result, it is possible to further increase the number of spreading codes to increase an amount of data concurrently transmissible, and to make fine adjustments to resources. In another aspect of the CDMA transmission apparatus of the invention, an inverse discrete Fourier transformer is applied to the spreading code generator and the spreader. According to this configuration, it is possible to perform spreading processing using spreading codes expressed in Eq. (1) with a small amount of calculation. In still another aspect of the CDMA transmission apparatus of the invention, a plurality of cascaded inverse discrete Fourier transformers is applied to the spreading code generator and the spreader, and performs inverse discrete Fourier transform on a transmission signal hierarchically. According to this configuration, it is possible to reduce an amount of calculation when spreading processing is performed using spreading codes with different spreading-factors while hierarchically using the spreading codes as expressed in Eq. (1). An aspect of a CDMA reception apparatus of the invention adopts a configuration provided with a spreading code generator that generates a bth chip C(a,b) of an ath spreading code by a following equation when it is assumed that e is a base of natural logarithm and N is a length of the spreading code (i.e. spreading code length):
In another aspect of the CDMA reception apparatus of the invention, a discrete Fourier transformer is applied to constitute the spreading code generator and the despreader. In still another aspect of the CDMA reception apparatus of the invention, a plurality of cascaded discrete Fourier transformers is applied to the spreading code generator and the despreader, and performs discrete Fourier transform on a received signal hierarchically. As described above, according to the present invention, it is possible to generate orthogonal spreading codes with arbitrary lengths. As a result, versatility is increased in frame length, basic clock and the like in a CDMA system, and it is thereby possible to facilitate the design of the CDMA system. This application is based on the Japanese Patent Application No. 2003-272882 filed on Jul. 10, 2003, entire content of which is expressly incorporated by reference herein. The present invention is suitable for use in a cellular telephone, base station and the like, for example. Referenced by
Classifications
Legal Events
Rotate |